This invention relates to a lens driving device and, in particular, to a lens driving device capable of moving a lens holder (a moving portion) holding a lens assembly (a lens barrel) in an optical direction of a lens.
A portable compact camera is mounted in a camera-equipped cellular mobile phone. The portable compact camera is provided with an autofocus lens driving device. Previously, various autofocus lens driving devices have been proposed. As a driving source (a driving method) used in such a lens driving device, a VCM scheme using a voice coil motor (VCM) is known.
A conventional lens driving device of the VCM scheme comprises, as the driving source (a driving portion), a driving coil and a magnetic circuit which comprises a yoke and a permanent magnet. The lens driving device is also called an actuator (e.g. see, Japanese Unexamined Patent Application Publication No. 2007-271878 (JP-A-2007-271878) (which will be also called Patent Document 1)).
More specifically, the conventional lens driving device of the VCM scheme drives the lens assembly (the lens barrel) using the VCM with a steplessly variable fashion between an infinity position and a macro position. Normally, the driving coil is fixed to a lens holder holding the lens assembly (the lens barrel). In a case of such a structure, the lens driving device of the VCM scheme becomes a moving-coil scheme. The lens driving device of the moving-coil scheme has a configuration in which the lens holder is driven by force based on Fleming's left-hand rule (Lorentz force) and the lens holder moves in the optical direction of the lens according to relationship (interaction) between a first magnetic field due to the magnet and a second magnetic field generated by flowing an electric current through the driving coil located in the first magnetic field.
As disclosed in Patent Document 1, the magnetic circuit in the conventional lens driving device of the VCM scheme includes the permanent magnet as an essential component.
As known in the art, inasmuch as the permanent magnet contains a rare metal as a composition, the magnetic circuit is expensive. In addition, inasmuch as the conventional lens driving device of the VCM scheme requires the permanent magnet, it is difficult to become smaller in size.
On the other hand, a thrust force becomes smaller in the conventional lens driving device of the VCM scheme with miniaturization of the lens driving device of recent years.